Hydrochemical and isotopic baselines for understanding hydrological processes across Macquarie Island

Isotopic and hydrochemical data from lakes provide direct information on catchment response to changing rainfall, evaporation, nutrient cycling, and the health of ecosystems. These techniques have not been widely applied to lakes in the Southern Hemisphere high latitudes, including Southern Ocean Islands (SOIs) experiencing rapid, significant shifts in climate. Historical work has highlighted the localised nature of geochemical drivers in controlling the hydrochemical evolution of lakes, such as geology, sea spray contribution, vegetation, geographical location, and ice cover extent. The role of groundwater in lake hydrology and hydrochemistry has not been identified until now, and its omission will have major implications for interpreting soil–water–air processes affecting lakes. Here we present the first comprehensive, island-wide hydrochemical and isotopic survey of lakes on a SOI. Forty lakes were examined across Macquarie Island, using comparable methods to identify key environmental processes and their geochemical drivers. Methods include stable carbon (δ13CDOC: dissolved organic carbon and δ13CDIC: dissolved inorganic carbon), oxygen (δ18O), hydrogen (δ2H) and strontium isotopic ratios (87Sr/86Sr) in water. These provide essential baseline data for hydrological, biological, and geochemical lake processes. Lakes on the western side of the island are influenced by sea spray aerosols. In general, it was found that lakes at higher elevations are dilute and those located in lower elevation catchments have experienced more water–rock interactions. The hydrochemical and isotopic tracers suggest that lakes in lower elevations contain more terrestrial sourced ions that may be contributed from groundwater. Increasing temperatures and changing rainfall patterns predicted for the region will lead to shifts in nutrient cycles, and impact the island’s unique ecosystems. Future research will focus on long-term monitoring to understand seasonal, annual, and long-term variability to test fundamental hypotheses concerning ecosystem function and the consequences of environmental change on SOIs.

www.nature.com/scientificreports/ While isotopes have not been used widely across the SOIs, lake water chemistry surveys have applied traditional hydrochemical and biological approaches that highlighted the localised nature of geochemical drivers controlling the hydrochemical evolution of lake waters such as geology, sea spray contribution, vegetation, geographical location, and ice cover extent 7,[12][13][14][15][16][17][18][19] . The role of groundwater in lake hydrology and hydrochemistry has not been identified, and its omission could have major implications for interpreting soil-water-air processes affecting lakes, and ultimately influencing how palaeoclimate lake records are interpreted. Understanding how the hydrology of the SOIs will change due to a changing climate is critical for predicting catchment and ecosystem response to future changes in rainfall, evaporation, and groundwater exchange.
One particularly vulnerable island in the region, is Macquarie Island (Fig. 1c). It is small (124 km 2 ), lowlying (maximum height of 433 m above sea level (a.s.l.) with low topographic variation, and is experiencing significant changes in multiple climate parameters such as rainfall 10 . As a result, the biodiversity of the island is under significant threat. For example, changes in water availability or hydrology of the island are suggested to be a primary cause of the widespread decline in the endemic, and now threatened, keystone cushion plant Azorella macquariensis 9,10,20 . The loss of the island's iconic vegetation terracing is, as a result, also of concern.
The main purpose of this study is to focus on new hydrological findings by using a variety of isotopic techniques that have not yet been applied to SOIs. In this study, these techniques were used to understand the role of rainfall, sea spray aerosols (SSAs) and groundwater inputs in governing the lake water chemistry from 40 lakes across Macquarie Island. We test whether the isolation of Macquarie Island from large continental landmasses including the influence of the Southern Ocean is the dominant process driving the geochemistry of lakes across Macquarie Island. We also explore whether terrestrial processes such as the influx of terrestrially sourced ions from groundwater plays a significant role in the lake water chemistry. To do this we used sophisticated isotopic methods such as stable carbon including both dissolved organic carbon (δ 13 C DOC ) and dissolved inorganic carbon (δ 13 C DIC ), oxygen (δ 18 O), hydrogen (δ 2 H) and strontium isotopic ratios ( 87 Sr/ 86 Sr) to provide necessary information on geochemical processes that cannot be ascertained by only measuring the concentration of ions in surface water 3 . As such, it provides essential baseline data for understanding how changing climate variables such as rainfall and temperature for the region will impact the hydrological, biological, and geochemical processes in SOI lakes.

Environmental setting
Macquarie Island (54°30′S, 158°57′E) is located in the Southern Ocean just north of the Polar Front (Fig. 1). It is 1500 km south-east of Tasmania, Australia, 1200 km southwest of New Zealand and 1500 km from the Antarctic continent. The island lies on the Australia/Indian and the Pacific tectonic plates, which are tectonically active 21 . It represents a rare example of uplifted oceanic crust that is part of the Macquarie Ridge Complex 22 and emerged ca. 600,000 years ago 23 . Most of the island is composed of pillow basalt with faulting and dolerites. The north contains ultrabasic, gabbro and troctolites rock types 24 . Recent geological units include the palaeo-beach and lake deposits, alluvium, colluvium and peat, which occur along the coast 25 . The island has steep coastal slopes rising to a plateau 200-300 m a.s.l.. Faulting, uplift, sea-level changes, erosion and periglacial processes have shaped the surface and its lakes 26 . Widespread glaciation did not occur during the Last Glacial Maximum, although it is possible that perennial ice and snow accumulated in some areas 27 . Drainage features and extents of modern lake systems are shown in Fig. 1c. Lakes on the island have not been observed to freeze completely, but up to 10 cm of ice cover has occurred on small ponds 26 . The soils were found to be derived largely from the underlying igneous material 21 .
The Island has a cool oceanic climate with mean temperatures between 3.1 °C in winter to 6.6 °C in summer from 1948 to 2020 28 . The mean annual precipitation is 992 mm (1948-2020) with rainfall on ~ 316 days of the year. Due to almost constant cloud cover, light levels are generally low, with a mean annual average of 2.4 h of sunshine per day 28 . The island is vegetated at lower elevations with tussock grasslands, herbs and sedges, mosses, liverworts and lichens 10,26 and there are no tall shrubs or trees 26 . All indigenous biota have colonised via longdistance dispersal 10 . Introduced vertebrates such as rabbits and rodents 29,30 have had devastating impacts and led to unprecedented changes to the island 30 . However, since the successful eradication program which commenced in 2010 and was completed in 2014, the island has shown significant signs of recovery 31 .

Methods
Field sampling and major ion analyses. Lake waters were obtained from 40 lakes during the Austral summer of 2018 (Fig. 1c). Lake water sample locations were chosen based on spatial distribution across the island and logistical feasibility. Water samples were collected from 20 cm below the lake surface and field parameters (electrical conductivity (EC), oxidation-reduction potential, dissolved oxygen (DO), temperature, and pH) were measured. Water samples were collected and filtered through a 0.45 μm polyethersulphone high-capacity filter. Full details of the methodology for surface water sample collection are provided in Meredith et al. 3 . Anions and cations were analysed using inductively coupled plasma mass spectrometry (ICP-MS), inductively coupled plasma-atomic emission spectrometry (ICP-AES) and ion chromatography (IC) ( Table S1).
Environmental isotopes and dissolved organic carbon concentrations. The stable oxygen and hydrogen (δ 18 O and δ 2 H) isotopes were analysed by a Picarro L2130-i Cavity Ring-Down Spectrometer and reported as per mil (‰) deviations from the international standard V-SMOW with an analytical precision of ± 0.2 and ± 1.0‰, respectively. The stable carbon isotope values of dissolved inorganic carbon (δ 13 C DIC ) values of waters were analysed by isotope ratio mass spectrometry (IRMS). After injecting the CO 2 into a helium stream, which was separated from other gases by gas chromatography, it is attached to a Finnigan 252 mass spectrometer using a Conflo III. Results were reported as ‰ deviation from the international carbonate standard, www.nature.com/scientificreports/ NBS19 with a precision of ± 0.1‰. The dissolved organic carbon (DOC) and δ 13 C DOC values were analysed using a total organic carbon analyser interfaced to a PDZ Europa20-20 IRMS utilising a GD-100 gas trap interface. Results were reported as ‰ deviation from the NIST standard reference material with an analytical precision of ± 0.6‰. Strontium (Sr) isotopic ratios ( 87 Sr/ 86 Sr) from 10 of the lakes (LK2, LK6, LK14, LK29, LK31, LK34, LK37, LK38, LK43 and LK44) (Fig. 1c) were measured at the University of Melbourne using a Nu Plasma multi-collector ICPMS (MC-ICPMS) equipped with a CETAC Aridus desolvator and low-uptake Glass Expansion nebuliser (approximately 0.07 ml min −1 ). Approximately 20 g of water was evaporated in a HEPA-filtered fume hood. The Sr was then extracted using a single pass over a 0.15 millilitre column of EICHROM Sr resin 32 . Blanks were run alongside the samples with new resin used for each sample to eliminate memory issues. A 2% nitric acid was used to dissolve the dried Sr and obtain a concentration of 40-45 parts per billion. Krypton, rubidium, and strontium were corrected for cone memory and rubidium interference using gas blanks. Data are corrected for drift using standard SRM987. Any instrumental mass bias was removed by internal normalisation to 88 Sr/ 86 Sr = 8.37521 using the exponential law. Internal precision of the mass bias corrected 87 Sr/ 86 Sr is between 1.2 × 10 -5 and 2.0 × 10 -5 . The estimated reproducibility (2σ) of standards and other materials on the multi-collector inductively coupled plasma mass spectrometer is estimated to be 4.0 × 10 -5 .
Mapping and statistical analyses. Shapiro-Wilk tests for normality returned p values < 0.05, indicating that distributions of parameters are significantly different from normal. Statistical relationships between variables were therefore assessed using the non-parametric Spearman's rank correlation coefficient (⍴). A Geographical Information System (GIS) was developed in ArcMap 10.2.1 in the coordinate system WGS 1984 UTM zone 57S. Each parameter measured was added to the GIS to assess the spatial distribution of parameters across the island (Figs. S1-S11). Principal Component Analysis (PCA) was performed with parameters that were standardised by subtracting the mean and dividing by the standard deviation using the prcomp() function in R (v.1.1.456 33 ). The following hydrochemical (in mmol l −1 ), isotopic (‰) and environmental parameters were used; distance to west coast (km), elevation (m), temperature (°C), DO (mg l −1 ), EC (µS cm −1 ), pH, Cl, SO 4 , SiO 2 , Na, Ca, K, Sr, Fe, Mn, δ 18 O, δ 2 H, δ 13 C DIC , DOC, δ 13 C DOC , F and Al. A hierarchical cluster analysis was used to group lakes based on their loadings on the PCA components using the HCPC() function (FactoMineR package 34 To determine likely sources of Sr for each sample, a second PCA was performed to see which variables clustered with high and low 87 Sr/ 86 Sr values.

Results
Field parameters and major ions. The results of the hydrochemistry and environmental isotopes for the 40 lakes are presented spatially in Figs. S1-S11 and are located in Tables S1 and S2.
The lake waters are oxic (8.6-12.6 mg l −1 ) and range from slightly acidic (pH 6.0) to slightly alkaline (pH 9.2). Lake water temperatures are generally highest for lakes along the west coast (greater than 10 °C, Table S2). Phosphate concentrations are below detection level (0.1 mg l −1 ) for all lakes and nitrate was low ranging from below detection limit (< 0.05) to 0.21 mg l −1 . A number of lakes have similar ionic ratios to seawater with Na-Cl type waters being the dominant cation and anion. Bicarbonate concentrations were calculated by difference and were low with a maximum of 36 mg l −1 and average of 3.7 mg l −1 . All lakes are low in Cl concentrations ranging from 1.4 (LK22) to 3.7 (LK3) mmol l −1 and Na concentrations ranging from 0.99 (LK28) to 2.57 (LK3) mmol L −1 . The SO 4 , Cl, Mg and Na concentrations follow a similar pattern to seawater in the Schoeller plot (Fig. 2), with all showing very high significant positive correlations (⍴ ≥ 0.75, p ≤ 1.4 × 10 -5 , Table S3). The K, Ca, F and Si concentrations follow a similar pattern to seawater for some lakes, whilst others diverge, with all lake waters containing higher Si concentrations than seawater (Fig. 2). An increase in Cl concentration is broadly reflected in the increase in Br, K and Sr (all ⍴ ≥ 0.71, p ≤ 1.7 × 10 -6 ). The high correlation between these variables implies a  Hierarchical cluster and principal component analysis. Hierarchical cluster analysis revealed three main clusters of lakes (Fig. 3). These clusters were used to colour code the results of the PCA to determine the main hydrochemical processes for the groupings. Cluster 1 represents lakes containing high SO 4 , Cl, Na, EC, Br, Sr, K, Mg, δ 2 H, Al and DOC concentration, and low pH, δ 13 C DIC and smaller distance from the west coast (all p < 0.5, Table S4). Cluster 2 represents lakes containing high Ca, F, SiO 2 , d-excess, pH and Fe, low elevations, K and δ 2 H (all p < 0.5, Table S4). Cluster 3 represents lakes containing low Na, EC, F, Cl, Ca, SO 4 , Fe, Mg, DOC concentrations, Br, SiO 2 , Al and are located at higher elevations (all p < 0.5, Table S4). PCA results show two main components explaining a total of 63% of the variability in the dataset. Components 1 and 2 of the PCA explain 41.9% and 21.1%, respectively. Variables loading most strongly on the first component (Dim1) include K, SO 4 , Sr, d-excess, Cl, δ 18 O, δ 2 H, Na and Br (Fig. S12). Variables loading most strongly on the second component (Dim2) include Ca, F, Fe, SiO 2 , DOC concentration, EC, elevation, Na and Mg (Fig. S12) Table S3). Overall, there is a general trend of decreasing Cl, Na, SO 4 and Sr with increasing distance from the west coast, whilst elements such as Fe, Ca, F and Si and isotopes such as δ 18 O, δ 13 C DIC and δ 13 C DOC do not show a clear trend (Fig. 5e,h,i,k,l). Sixteen of the lakes have SiO 2 values of 0.001 mmol l −1 , whilst LK16 contained the highest concentration (0.087 mmol l −1 ). LK16 is located at the second lowest elevation (at 95 m a.s.l) of lakes in this study (Table S2). Ca concentrations range from 0.02 mmol l −1 (LK8) to 0.29 mmol l −1 (LK9). LK8 is located at the third lowest elevation of lakes sampled in the study (Table S2). Three lakes have F concentrations of 0.0003 mmol l −1 , whilst the highest F concentration is identified in LK16 at 0.0058 mmol l −1 . SiO 2 , Ca and F are not significantly related to distance from the west coast (⍴ = 0.10, 0.22 and 0.08, p = 0.52, 0.18 and 0.61 respectively) (Fig. 5h,f,g) or Cl concentration (⍴ = 0.16, − 0.14 and 0.23, p = 0.33, 0.37 and 0.15 respectively). Fe and F are, however, significantly negatively correlated with elevation (⍴ = − 0.49, p = 1.5 × 10 -3 and ⍴ = -0.41, p = 9.4 × 10 -3 respectively). Ca and F are positively correlated with SiO 2 (⍴ = 0.78 and 0.49, p = 2.4 × 10 -9 and 1.3 × 10 -3 respectively, Table S3) suggesting that their source is different from that of SO 4 , Cl, Mg and K (see also Fig. 2).

Discussion
The following discussion provides new hydrological findings for lakes across Macquarie Island based on hydrochemical and isotopic measurements. Macquarie Island is a remote field location, and the rainfall has not been sampled or measured for isotopes or hydrochemistry, and neither has the physical hydrology or hydrogeology of the aquifers been investigated. Inference of the rainfall isotope composition for Macquarie Island is made based on global data. This is because the nearest high-resolution rainfall and aerosol collection site is located at Cape Grim on the north west coast of Tasmania 37 (Fig. 1b), which is approximately 1800 km north west of Macquarie Island. Considering these limitations, the lake hydrochemistry of Macquarie Island is explored in the following discussion.
Hydrological findings. The underlying geology and its structures control surface water and groundwater expressions across the island. Lakes across the island have not been observed to freeze completely, but ice cover can occur on small ponds. The lakes range from closed to open systems and do not show evidence of overflow. Groundwater springs have not been mapped across the island, but the elevational changes and Landsat images suggest surface expressions related to groundwater discharge sites. The δ 2 H and δ 18 O values of lake waters do not suggest a snowmelt origin as found across the Signy Island 11 or a seawater origin, despite a strong SSA input (Fig. 4). Instead, lakes were slightly more enriched in δ 18 O (by over 1‰) but had similar δ 2 H values compared to the amount weighted rainfall values observed at Cape Grim 36 (Fig. 4). However, this enrichment trend is observational and needs to be tested with further studies because Cape Grim is located ~ 1800 km northwest of Macquarie Island, and the localised rainfall patterns may differ greatly. The higher δ 18 O values suggest evaporation of the lakes, which may be related to the area of the lake, suggesting smaller lakes are experiencing more evaporation. These findings show that the major source of water to lakes across Macquarie Island is from a rain- www.nature.com/scientificreports/ fall source that may be from direct precipitation into the lake catchment or by rainfall that has been recharged as groundwater into the underlying bedrock aquifer. The lakes can be broadly grouped into three clusters based on hydrochemistry and isotopic compositions (Fig. 6). Cluster 1 is associated with lakes on the western side of the island and while dilute in concentration compared to seawater, they contain higher concentrations of Na, Cl, Mg, K and SO 4 , ions which are found in seawater 38,39 . The highest concentrations of these ions occur along the west coast (LK3, 6,10,18,37,38,44) and decrease in concentration with increasing distance from the west coast (Fig. 7). Within Cluster 1, four samples (LK6, 37,38,44) were analysed for 87 Sr/ 86 Sr isotope ratios (0.70866, 0.70894, 0.70908, 0.70904, respectively) and were close to the isotopic ratio of seawater 87 Sr/ 86 Sr (0.709, Fig. 5j). LK6 had a slightly lower value suggesting a lighter source of Sr for this lake.
Between 370 and 2423 kg per hectare per year of NaCl is deposited over Macquarie Island 40 . The lakes accumulate salt over time, with concentrations likely to be dependent on the distance to the ocean, dilution from rainfall and evaporation. SSA contributions to lakesalong the west coast, match previous lake investigations that found that the main environmental process influencing lake water chemistry is the input of SSA; controlled by the prevailing westerly winds and the distance of the lake from the ocean on the western side of Macquarie Island 7,12-16 , however this process does not explain the hydrochemical variations observed for lakes in Clusters 2 and 3.
The PCA analysis also explains terrestrial processes associated with water-rock interaction and they are associated with elevation. Lakes increase in Ca, F and Si concentration with distance from the west coast (Fig. 5f,g,h). Figure 5. Distribution of ion concentrations and isotopes relative to distance from the west coast. Note the generally high concentrations of (a) Cl, (b) Na, (c) SO 4 and (d) Sr in the lakes from Cluster 1 (green) are located close to the west coast. Concentrations of (e) Fe are mixed in lake waters impacted by sea-spray, whilst concentrations of (f) Ca, (g) F and (h) SiO 2 are low in these lakes and increase with distance from the west coast, particularly in samples identified as having undergone high water-rock interaction. Lakes in Cluster 1 located close to the west coast contain higher (i) δ 18 O and (j) 87 Sr/ 86 Sr ratios and relatively low (k) δ 13 C DIC values. δ 13 C DOC values (l) show no significant relationship with distance from the west coast (p > 0.05).  Fig. 4). Cluster 3 contains lakes located at higher elevations (~ 150-300 m asl) with terrestrially sourced solutes but at lower concentrations than Cluster 2. Dissolved organic carbon concentrations are lower in the higher elevations but the δ 13 C DOC values were generally close to soil organic matter values (− 24.7 to − 29.2‰ 21 ) and within the freshwater dissolved organic carbon, particulate organic carbon and algae range 41 . The similarity between soil organic matter and DOC values in lake waters suggests limited in situ biological processing in the lakes.  www.nature.com/scientificreports/ Terrestrially derived solutes are most likely to be from the weathering of the underlying volcanic bedrock, which is predominantly basalt 24 . Lower elevation lakes have higher concentrations of terrestrially sourced solutes such as F, Ca and Si (Fig. 8a-c) due to increasing water-rock interaction (Fig. 8d,e). The highest Fe concentrations were associated with terrestrial sourced samples (Cluster 2) at the lowest elevations (< 100 m a.s.l.). This suggests that more dilute lakes at higher elevations are rainfall fed where waters from Cluster 2 and 3 sit closer to the GMWL (Fig. 4). Higher salinity waters containing terrestrial solutes located at the lower elevations are likely to be from either the flow of rainfall across the landscape during larger rainfall events resulting in water flowing from higher to lower elevations within localised lake catchments, or from groundwater influx from the underlying bedrock aquifer where the water has undergone greater water-rock interaction processes (Fig. 7). The later is a more likely explanation given that groundwater input can represent up to half of the annual water inputs and in some cases can be the primary source of ions into closed lake systems 42 . 87 Sr/ 86 Sr ratios can be used to trace groundwater 43 and can be particularly useful in the identification of Sr from bedrock sources (silicate weathering versus carbonate weathering 5 ) due to their different isotopic ratios. The decay of 87 Rb into 87 Sr, which is incorporated into silicate minerals due to the substitution of Rb in K-bearing minerals, results in high 87 Sr/ 86 Sr ratios in waters that have interacted with silicates (up to 0.7625 in silicates 44 ) compared to seawater. In contrast, young oceanic basalts, such as those comprising Macquarie Island, contain low 87 Sr/ 86 Sr ratios (approximately 0.702-0.705) due to their low Rb/Sr ratios and lower radioactive 87 Rb decay compared to older rocks 45 . The 87 Sr/ 86 Sr ratios of lake waters from Cluster 3 range from 0.70694 (LK29) to 0.70877 (LK43), which is slightly higher than the bedrock value. LK29 with the lowest 87 Sr/ 86 Sr ratio also has an enriched δ 13 C DIC value (− 1.5‰), which likely indicates carbonate dissolution. The negative relationship between δ 13 C DIC and 87 Sr/ 86 Sr suggests dissolution of carbonates associated with basalts (Fig. 8f). This may be in the form of carbonate veins, which have previously been identified in Macquarie Island peridotites 46 . This trend is supported by the relationship between Sr, Ca (Fig. 8g) and Si (Fig. 8h). However, δ 13 C DIC values for LK14 (− 13.9‰) and LK31 (− 11.2‰) do suggest dissolved inorganic carbon from a groundwater source, even though 87 Sr/ 86 Sr ratios are much higher than the oceanic basalt values of 0.702-0.705. Whether the Sr is sourced from carbonate contained in the bedrock or the bedrock itself, these findings are significant showing that these lake waters likely have groundwater inputs. Lakes are coloured by cluster with red, green and blue points representing lakes with high sea-spray input and samples which have undergone high and low water-rock interaction (cluster groups 1, 2 and 3 respectively in Fig. 3a). Notably, the highest 87 Sr/ 86 Sr values (n = 10) in the dataset are associated with sea-spray inputs including SO 4 , Br, Cl, Na and conductivity. www.nature.com/scientificreports/ Implications. Baseline isotopic and hydrochemical values of lakes across Macquarie Island presented in this study provide fundamental information for further studies to focus on quantifying how changing rainfall patterns driven by climate change may affect lake water composition, and evaporation. This is particularly important given that current climate projections under a range of 1.5-4 °C global warming scenarios suggest temperature increases of approximately 1.5 °C-2 °C and precipitation increases of approximately 10%-60% at Macquarie Island 47 . Episodic increases in the proportion of lake water coming from precipitation during extreme events will likely result in periods of high rainfall leading to strong dilution and greater solute concentration ranges in higher elevation lakes. In contrast, increased groundwater table height in the underlying aquifer because of increased precipitation may increase terrestrially derived ions from groundwater inputs or overland flow in lower elevation lakes. Previous studies have identified that increased sunshine hours and a change in rainfall patterns are leading to large, episodic rainfall events, resulting in a reduction in plant water availability 9,10,20 and likely shifts in lake water nutrient concentrations due to changes in vegetation cover 20 . Macquarie Island hosts unique ecosystems and with warming temperatures and changes in rainfall patterns this will impact the hydrology of lake catchments. The transportation of nutrients via overland flow processes will influence the growth and distribution of unique vegetation. Understanding the sources of water and the processes affecting current lake water chemistry, and how these may change in a warming climate, provide insights into potential impacts on the ecology of the island and its lakes due to climate change.

Conclusion
This study is the first to show that terrestrial sourced ions most likely from groundwater is a major contributor to lakes across Macquarie Island. Lakes on the western side of the island are primarily influenced by SSAs. Lakes at higher elevations are generally dilute and those located in lower catchments have experienced more water-rock interactions as water moves from higher to lower elevations. We are unable to conclusively prove whether the water comes from overland flow, groundwater or sub-surface flow with this dataset. Future hydrological research on Macquarie Island and other SOIs will focus on long-term monitoring of lake and rainfall to understand seasonal, annual, and long-term variability and change. This research is required urgently because lake systems have unique environments and ecosystems that are facing unprecedented pressure from climate change. Long-term multi-disciplinary research including baseline surveys are needed to test fundamental hypotheses concerning ecosystem function and nutrient cycling on Macquarie Island and other SOIs.

Data availability
The full dataset used in this study is available upon request to the corresponding author.

Code availability
The code used to develop individual figures is available upon request to the corresponding author.